Two general approaches typically are used to intensity-modulate light: direct modulation and external modulation.
In a direct modulation approach, a laser (e.g., a laser diode) is directly modulated by an information signal to generate a modulated laser output. The laser output power often is modulated directly by modulating the input drive current to the laser. The laser begins lasing when the drive current exceeds a threshold current level. Typically, the modulation range of input drive current that is applied to a directly modulated laser extends above and below the threshold current level.
In an external modulation approach, a modulator modulates the intensity of light generated by a continuous wave laser in accordance with an information signal. The modulator and laser may be disposed on separate, discrete substrates or they may be fabricated together on a single substrate. External modulators fall into two main families: electro-optic type modulators, such as Mach-Zehnder type electro-optic modulators, which modulate light through destructive interference; and electro-absorption modulators, which modulate light by absorption (e.g., through the Quantum Confined Stark effect). The absorption depth (i.e., the extinction ratio) and the spectrum of an electro-absorption modulator depends on the drive voltage across the modulator. An external modulator typically is positioned to receive the output of a continuous wave laser and the voltage across the modulator is varied to produce a digital optical bit stream.
In an external optical modulator, a time-varying electric signal modulates the input optical signal. Lump-type (e.g., electro-absorptive) external optical modulators typically are limited at high frequencies by their total capacitance. Typical high-speed external optical modulators avoid such a limitation by using a traveling-wave electrode structure, which includes a transmission line signal electrode located near an optical waveguide carrying the input optical signal. In a traveling wave optical modulator (e.g., a Mach-Zender interferometer), the optical modulation is dominated by the distributed interaction between the time-varying input electrical signal and an optical signal over the length of the optical modulator. Ideally, the input electrical signal and the input optical signal propagate with substantially the same phase velocities through a traveling wave optical modulator so that each portion of the optical signal interacts with substantially the same portion of the applied electrical signal as it propagates through the modulator.
In addition to matching optical and electrical phase velocities, it is desirable to reduce electrical losses as the input electrical signal travels through a traveling wave optical modulator. To this end, it is desirable to match the characteristic impedance of the optical modulator to the impedance of the source of the input electrical signal to reduce reflections and increase the electrical voltage delivered across the optical modulator. The geometry of the input signal electrode may be tailored to match optical and electrical phase velocities, to match the characteristic impedance of the modulator to the source impedance, or to reduce electrical losses along the signal electrode arising from impedance mismatch between the source impedance and the modulator. Typical electrode geometry parameters that are varied to achieve the desired electrical parameters include the thickness of the electrode, the width of the electrode, and the spacing between the top electrode and the side ground electrode (if present).